There is an increasing need for a simple and easily usable means whereby individuals most at risk from bone fracture due to osteoporosis, that is to say net loss of bone mineralisation, may be identified. Elderly persons are one of the groups most at risk of this condition and, as the average life expectancy increases, the incidence of bone fractures due to osteoporosis in the general population is increasing also.
Various devices are known for measuring bone density, including x-ray, gamma photon apparatus, and ultrasound apparatus. The latter are particularly advantageous in that they are basically non-invasive.
In the use of ultrasound, it has been known that the velocity of the signal through a patient's bone is a measure of the elasticity and density of the bone. However, it has more recently been found that by measuring broadband ultrasonic attenuation (BUA) there can be obtained data which is a measure of both the density and structure of cancellous bone. The data is expressed as the increase in ultrasonic attenuation with frequency in the range 200 kHz to 600 kHz.
One such device measures ultrasonic attenuation as ultrasound is transmitted through a bath of water, first without and then with a patient's foot immersed in the water. The data derived from the test is a measure of bone ultrasound attenuation.
A second such device, described in UK patent application GB 2257253 (published 6th Jan. 1993), includes a pair of ultrasonic transducers which, in use, contact the patient thus avoiding the need for a water bath. A silicone pad is attached to each transducer which, in use, contacts the patient's body part.
In the device of GB 2257253 one of the transducers is fixed and the other of the pair is movable. A patients body part e.g. heel, is placed against the fixed transducer and the movable transducer is then moved into contact with the body part. This allows the measurement to be taken.
One possible disadvantage with this apparatus is that the accuracy with which measurements can be repeated is limited. The pressure of the patient's body part against the fixed transducer may vary, the contact with that transducer (or indeed the other movable transducer) may also vary and the position of the patient's body part may change. Additionally, the pressure exerted by the movable transducer varies according to the size of body part i.e. according to the spacing between the transducers. Since the pressure exerted by the transducer affects the compression of a silicone pad on the transducer--and the ultrasound calculations made typically assume a constant compression in order to calculate the distance between the transducers--then this will affect the accuracy and repeatability of the measurements.